The move could prove to be a sea change for local economies, policing efforts, governments, schools, workplaces — you name it.

But what about the U.S. border? What should Canadians know if they're crossing over after marijuana is legalized?

To answer this question, HuffPost Canada reached out to Len Saunders, an attorney based in Blaine, Wash. who regularly works with clients barred or denied from entering the U.S. over marijuana-related issues.

Here are some of his tips for travellers crossing the border after weed becomes legal:

You most definitely cannot bring marijuana with you across the border

The federal government has made it abundantly clear that even though pot will be legal in Canada on Oct. 17, it is absolutely, totally, 100 per cent not allowed to be taken out of the country.

Even if you accidentally forgot some in your car, Saunders says, officers could slap you with a hefty fine and a lifetime ban from the U.S.

You can't bring pot back from the U.S., either, even if you're coming back from a state that has legalized it like Washington State or Colorado.

If for some reason you do have cannabis with you or in your car — please don't do this — while entering Canada or the U.S., declare it to the Canada Border Services Agency.

U.S. border guards are likely to ask Canadians more about pot once it's legal

Right now, the only border states with Canada that have legalized pot for recreational use are Washington State, Alaska, Vermont and Maine.

Saunders says it's likely that border officers at these crossings will be more keen to ask travellers about their pot use or involvement in cannabis. But once pot is legal everywhere in Canada, he noted, guards at every single border crossing could become extra vigilant.

Saunders said he expects a "tidal wave" of cases of Canadians getting barred over marijuana related issues.

Officers could pull you aside for more questions if you make them "suspicious"

The federal government says the U.S. has made it clear that it's business as usual at the border when it comes to marijuana-related inspections.

Stevie O'Brien, chief of staff to Border Security Minister Bill Blair, told HuffPost Canada that U.S. officers might pull a traveller aside for further questioning if they're given "reason to be suspicious."

So, what constitutes a "reason" to be suspicious?

"It will be a controlled substance at that thin line ... called the U.S. border."

"When you approach that border, don't give the Americans the reason to believe, either by the aroma or flicking something away as you approach the border, don't give them a reason to believe there's an offence," Public Safety Minister Ralph Goodale told CTV in June.

You can still be banned from the U.S. if you admit to pot use or if you say you work in (legal) cannabis

You read that right. Just admitting to having used marijuana in the past could get you a lifetime ban from the U.S., Saunders said.

Even though some states have legalized it for recreational use, cannabis is still illegal in the eyes of the U.S. federal government.

"It won't be a controlled substance in Canada, and it won't be a controlled substance in many border states like Vermont and Washington State," he said, "but it will be a controlled substance at that thin line ... called the U.S. border."

The Canadian government's messaging on this so far is that travellers going south should always tell the truth if they're asked about their pot use. But Saunders called that "dangerous" advice, since it can result in a lifetime ban that will require the traveller to use a waiver every time they want to enter the U.S. — and that costs a lot of time and a lot of money.

More from HuffPost Canada:

You can choose to not answer questions about pot

Saunders said he regularly advises people to refrain from answering questions about pot use at the border. Not answering the question might get you turned away, he said, but it at least helps you avoid being barred from the U.S., not to mention the torturously expensive and complicated process required to get a permit waiving the ban.

"In order to avoid a life-time bar ... it's better to just not answer that question. They can't force you to answer that question," he said.

Saunders says officers might reject your entry and turn you back, but you won't be barred.

"You can go back to the border at any point and try a re-entry. Most officers, once you go back, ... are not going to pursue the marijuana thing. It's not like every officer is zealously trying to get Canadians to admit to marijuana smoking."

CLARIFICATION: A previous version of this story did not include Alaska in a list of border states with legal marijuana.